“Out-focus” is a photographing technique which highlights a main object concerned by positioning the main object within the depth of field and positioning the background out of the depth of filed to make the background blurred. This can be explained by a geometric optical method using a circle of confusion (COC).
The size k of the circle of confusion can be expressed by Equation (1) below, where N denotes an f-number, f denotes a focal length, u denotes a distance from a camera to a focus surface, and ud denotes a background distance.
                    k        =                              1            N                    ⁢                                    f              2                                      u              -              f                                ⁢                                                                                    u                  d                                -                u                                                                  u              d                                                          (        1        )            
In Equation (1), the size k of the circle of confusion differs in accordance with f number and the focal length of a camera even with respect to the same values of u and ud. Also, as can be understood from Equation (1), the size k of the circle of confusion is inversely proportional to the f-number, and in proportion to a square of f.
In general, as the ratio of the circle of confusion to the background increases, the out focus effect becomes more pronounced, whereas as the ratio of the circle of confusion to the background decreases, the out-focus effect becomes less pronounced.
A method of implementing the out-focus effect in the related art is performed mainly using a DSLR (Digital Single-Lens Reflex) camera. The DSLR camera is a lens exchange type camera. Since the DSLR camera can make the f-number small and make the focal length long, the size of the circle of confusion and the ratio of the circle of confusion to the background can be increased, and thus the out-focus effect can be well presented.
However, in a DSC (Digital Still Camera) or a phone camera, the focal length is generally short and the f-number is generally large, and it may not be possible to properly present the out-focus effect. In comparison to the DSLR camera, the out-focus effect of a DSC or phone camera may be tens to several hundreds of times lower than that of the DSLR camera.